Two different sets of flight rules apply when piloting an aircraft, whether it is a private single-engine aircraft or multi-engine jet airliners. The two sets of rules are known as visual flight rules (VFR) and instrument flight rules (IFR). Visual flight rules mean pilots rely on external visual reference points they can see.
Pilots cannot fly VFR if they are flying through cloud or within defined limits of cloud because they must be able to see other aircraft. Air traffic control (ATC) is not necessarily responsible for separating aircraft that are flying VFR, although services such as flight tracking are available depending on the region.
Weather conditions dictate whether pilots fly VFR or IFR
Whether pilots fly VFR or IFR will depend in part on weather conditions, flight route and other variables. With respect to weather conditions, the Federal Aviation Administration (FAA) defines specific values for cloud ceiling and visibility in a rulebook known as the Federal Air Regulations (FAR). These vary depending on the type of flight and the airspace in which the aircraft is operating and are defined by FAR Part 91.155. In Class B airspace, there must be at least 3 statute miles of visibility and pilots must fly clear of cloud. In general, for controlled airspace, the restrictions are as follows,
|Airspace||Minimum visibility||cloud distance|
|Class A||Not applicable||Not applicable|
|Class B||3 statute miles||Sheltered from the clouds|
|Class C||3 statute miles||500 feet below 1000 feet above 2000 horizontal feet|
|Class D||3 statute miles||500 feet below 1000 feet above 2000 horizontal feet|
|Class E (||3 statute miles||500 feet below 1000 feet above 2000 horizontal feet|
|Class E (≥10,000 ft MSL)||5 statute miles||1.00 feet below 1000 feet above 1 horizontal statute mile|
With ATC clearance, it is possible to deviate from published minima under conditions known as “Special VFR Weather Conditions” (FAR Part 91.157).
You may have noticed that on the table above, the minimums for VFR flight in Class A airspace are not listed. In the United States, all airspace between 18,000 feet MSL and FL600 (approximately 60,000 feet above sea level) is Class A airspace. A from the United States must be IFR, with a few exceptions.
You need good weather to fly VFR. Photo: Getty Images
When flying IFR, pilots have a whole different set of concerns. Instead of searching for other aircraft, they rely on ATC to prevent them from colliding with other aircraft. However, an entirely different type of flight is required, which means pilots need additional certification to be able to fly IFR.
VFR flight is less restrictive than IFR flight
The way pilots fly differs significantly between VFR and IFR flight. When flying VFR, pilots rely on what they can see out the window. This is the purpose of the table of minimum visibilities and cloud clearances shown above. Referring to charts, pilots flying VFR can either plan a flight or simply fly where they please (as long as they don’t break any rules such as airspace violation).
This means that a pilot flying under VFR can plan his flight around VRPs or visual reference points. For example, you can follow the river to the town before following the highway to the next town. If the pilot sees a point of interest en route, nothing prevents him from deviating from his plan to take a closer look.
The reverse applies during an IFR flight. IFR flights must have a flight plan, which means that the origin, destination and route are filed in advance. Pilots are then expected to follow the instructions of the air traffic controller they are communicating with, and if they stray from their path, questions will be raised.
When flying IFR, you must do what ATC tells you. Photo: Getty Images
When pilots learn to fly, they start by learning to fly VFR, relying on the horizon and what can be seen from outside. After earning their Private Pilot Certification (PPL), many pilots will then earn an additional certification that will essentially require them to start from scratch.
When learning IFR flight, pilots often wear special glasses that allow them to see the instruments but not what is happening outside the aircraft during lessons. While studying for their PPL, pilots in the United States must complete three hours of flight training with reference only to instruments. This means that if they accidentally find themselves in IFR conditions, they must have the basic skills necessary to exit and land safely.
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